As a practical matter, the eye is not spherical. Thus, in order to improve the fit and performance of contact lenses, lenses commonly include multiple surfaces. For example, many lens designs include a central zone with a periphery zone located about the circumference of the central zone. The term "zone" is intended to describe an area of the lens which may comprise a single or multiple surfaces. The central zone is designed primarily for vision correction, whereas the periphery zone is commonly designed primarily for fit. Specific examples of such lenses are provided in the following patents, each of which is incorporated herein by way of reference. U.S. Pat. No. 4,883,350 to Muchenhirn discloses a contact lens having a back side including a spherically shaped central zone with an aspherically shaped periphery zone. U.S. Pat. No. 4,765,728 to Porat et al. discloses a lens having a back side including central and peripheral zones both having surfaces described by non-spherical second-order surfaces of revolution (i.e. non-spherical conic functions such as an ellipse, parabola and hyperbola). U.S. Pat. No. 5,436,678 to Carroll discloses a multifocal lens wherein the back side of the lens includes a central zone surrounded circumferencially by a marginal zone, which is surrounded circumferencially by a periphery zone. Each zone has a shape defined by a non-spherical conic function. U.S. Pat. No. 5,452,031 to Ducharme discloses a central optic zone with a peripheral zone defined by splines (piecewise polynomials linked to one another).
With lenses including multiple surfaces, it is often desirable to have adjacent surfaces be tangent with one another along their points of intersection. For example, the adjacent zones on the back side of the lens can be connected along common points which are tangent with the surfaces of the adjacent zones, as is described in the aforementioned references. By making adjacent zones tangent with one another, the transitions between zones are relatively smooth (junctionless), thus improving comfort, vision properties, and manufacturability.
In addition to the number and shape of surfaces on a lens, other lens design parameters must also be considered in making lenses, e.g. overall lens diameter, central zone diameter, sagital depth, lens power, base curve, axial edge lift, etc. Axial lift is the distance between a point on the back side of the lens (measured parallel to the lens major axis) at a specific diameter and a theoretical vertex sphere. The theoretical vertex sphere is a sphere having a radius equal to the lens base curve of radius.
Although many lenses designs are known, new and simplified approaches continue to be sought.